Plunger lift and safety valve system

ABSTRACT

A plunger lift production system uses a production string including a bottom hole assembly, a production tubing having an inside diameter and including a safety valve having an inside diameter, and a plunger with an outside diameter which closely matches the inside diameter of the safety valve, and an expandable lug and pad assembly, which when expanded closely matches the inside diameter of the production tubing, and when not expanded closely matches the inside diameter of the safety valve.

FIELD OF THE INVENTION

The present invention is directed to a system using an intermittent plunger lift and a subsurface safety valve.

BACKGROUND

As gas wells age, reservoirs deplete and gas velocities become insufficient to lift the fluids associated with production from the subterranean reservoir. The completion equipment alone can provide significant challenges in maintaining unloaded production since completion components can act as mechanical restrictions to the production enhancement equipment designed to optimize production.

Completion of a conventional plunger lift well consists of a no-go seal landing assembly equipped with a standing valve and a bumper spring disposed in the tubing string at a predetermined depth coinciding with the uppermost selective profile nipple. The surface equipment consists of a receiver or lubricator on surface to receive the plunger above the wellhead. The receiver is furnished with a manual or pneumatically actuated catcher to catch and hold the plunger at surface. The plunger lift system cycle is controlled using an electronic wellhead controller. The wellhead controller controls the signal to the motor valve which opens and closes the well to production as required and activates and releases the catcher to catch and release the plunger at surface in a timed sequence as programmed in the controller.

One completion component of note is the subsurface safety valve (SSSV). These valves are typically landed about 50 m to 300 m below the surface of the producing well. In many applications, these SSSVs are required to remain in an active state to provide well control in the event of an emergency, therefore, they cannot be disabled during the production operation. Some provisions exist including re-zoning of a producing well whereby the SSSV may be deactivated and production of the well can continue. In this case the SSSV remains intact but functionally disabled in the wellbore for the remainder of the well's producing life. Therefore the challenge becomes lifting the produced liquids to surface utilizing the existing wellbore configuration and thereby avoiding costly workover expenditures.

Other alternative workovers including capillary strings specially designed to pass around a specialty SSSV. These completions will also require workovers to pull and replace the SSSV with the special model for capillary string installation.

Therefore, in conventional completions, bumper spring assemblies are landed in the bottom of the well below the SSSV, and the plunger is cycled below the valve. In this method the plunger cycles without any control below the SSSV and therefore arrivals cannot be identified and cannot be optimized. This type of system results in uncontrolled cycling of the plunger between the bottom hole spring immediately below the SSSV and the lower most spring assembly landed at or near the end of the tubing string. Uncontrolled cycling of the tubing plunger can result in significant damage to many components of the plunger lift system.

As is well known in the art, a plunger allowed to free cycle in any tubing string in an uncontrolled fashion will lead to significant equipment damage due to uncontrolled free fall and rapid ascent of the plunger. The most detrimental component of this operation is the inability to control plunger cycles to ensure that a sufficient liquid slug travels above the plunger to cushion the arrival at the wellhead lubricator. Equally important is allowing a sufficient flow time between cycles to build a fluid column in the tubing string above the bottom-hole assembly. This fluid head serves to cushion the landing of the plunger on the bottom-hole assembly.

Flowing the well to its best potential requires that no significant back pressure should be present during the operation. The addition of a safety valve in the production tubing string acts as an obstacle to the efficient and consistent cycling of the plunger.

Therefore, there is a need in the art for a system which more efficiently utilizes a plunger lift system and a subsurface safety valve.

SUMMARY OF THE INVENTION

The invention relates to a plunger lift system for lifting produced fluids from a bottom hole spring assembly, wherein the plunger cycles through a modified safety valve to a wellhead receiver (lubricator) on surface.

In one aspect, the invention comprises a plunger lift production system comprising:

(a) a production string comprising a bottom hole assembly, a production tubing having an inside diameter and including a safety valve having an inside diameter;

(b) a plunger comprising a plunger body having an outside diameter which closely matches the inside diameter of the safety valve, and an expandable lug and pad assembly, which when expanded closely matches the inside diameter of the production tubing, and when not expanded closely matches the inside diameter of the safety valve.

In another aspect, the invention comprises a method of producing fluids from a subterranean wellbore having a production string comprising a bottom hole assembly, a production tubing having an inside diameter and including a safety valve having an inside diameter, the method comprising the step of cycling a plunger within the production string through the safety valve, wherein the plunger comprises a plunger body having an outside diameter which closely matches the inside diameter of the safety valve, and an expandable lug and pad assembly, which when expanded closely matches the inside diameter of the production tubing, and when not expanded closely matches the inside diameter of the safety valve.

Accordingly, the plunger is able to effectively sweep the produced liquids from the well bottom to top, through a safety valve.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like elements are assigned like reference numerals. The drawings are not necessarily to scale, with the emphasis instead placed upon the principles of the present invention. Additionally, each of the embodiments depicted are but one of a number of possible arrangements utilizing the fundamental concepts of the present invention. The drawings are briefly described as follows:

FIG. 1 shows a schematic of one embodiment of a production system.

FIG. 2A shows a partial cutaway of one embodiment of a plunger for use in a production system. FIG. 2B shows another embodiment of a plunger with a lug and pad assembly retracted. FIG. 2C shows an end view of the view of FIG. 2B. FIG. 2D shows the embodiment of 2B with the lug and pad assembly expanded. FIG. 2E shows an end view of the view of FIG. 2D. FIG. 2F shows a cross-sectional view along line 2F in FIG. 2B.

FIG. 3 shows partial cutaways of one embodiment of a subsurface safety valve in both open and closed configurations.

FIG. 4 shows a partial cutaway of another embodiment of a subsurface safety valve.

FIG. 5 shows a pressure trend graph in a prior art production system.

FIG. 6 shows a pressure trend graph with implementation of a production system and method of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention relates to a system and method of using a plunger lift with a subsurface safety valve. When describing the present invention, all terms not defined herein have their common art-recognized meanings. To the extent that the following description is of a specific embodiment or a particular use of the invention, it is intended to be illustrative only, and not limiting of the claimed invention. The following description is intended to cover all alternatives, modifications and equivalents that are included in the spirit and scope of the invention, as defined in the appended claims.

Bottom hole completions for producing wells using intermittent plungers are well known. Examples of such completion systems are described in U.S. Pat. No. 7,347,273 issued on Mar. 25, 2008, the contents of which are incorporated by reference (where permitted). These bottom hole completions are facilitated by providing a bottom hole assembly which can be deployed by slickline through a safety valve, and landed at or near the bottom of the tubing string.

To efficiently optimize a producing well using a plunger lift system the cycles must be controlled. Monitoring the plunger cycle for optimizing production requires analysis of the signature plunger arrival which can only be accomplished with recorded arrivals. The apparatus and system for monitoring and controlling the plunger cycle are well known in the industry and need not be further described herein.

A wellbore schematic is shown in FIG. 1. As a result of the completion, a bottom hole sliplock and spring assembly (10) is landed in the production tubular (12) in the wellbore. A thru-sleeve plunger (14) impacts the spring assembly at the end of its descent in the wellbore, resulting in the plunger being prepared to ascend in the production tubular under fluid pressure, carrying production fluids with it up to the surface.

Completions of the well in this fashion to maintain unloaded production is highly economic since the producers can avoid costly workovers involving pulling the tubing and removing a safety valve in favor of installing a wireline retrievable model.

The safety valve (30), one embodiment of which is shown in FIG. 3, is designed to be placed in the production tubing, to provide flow control in planned or unplanned shutdowns. The flapper valve (32) opens and closes a throughbore (34) which is sized to permit the plunger (14) to pass through. In one embodiment, the safety valve is tubing retrievable, as contrasted with wireline retrievable configurations. One embodiment of a tubing retrievable safety valve or TRSSSV (30) is shown in FIG. 4.

In contrast to a conventional method of completing a plunger lift well, a TRSSSV well equipped with a plunger lift will consist of a hold down selective in nature since the first restriction which it must pass through is the selective profile nipple within the TRSSSV. The selective nature of the landing assembly allows securing of the bottom-hole assembly and bumper spring at any desired depth below any number of existing selective profile nipples. This can be accomplished using a device such as that described in U.S. Pat. No. 7,347,273, the contents of which are incorporated herein by reference where permitted, or the like.

As shown in FIGS. 2A-2F, the plunger (14) is equipped with specialized pad mandrels (16) with spring loaded lugs (20) design to allow the lugs (20) and pads (18) to collapse to the outside diameter (OD) of the plunger body and thereby guarantee passage of the plunger through the safety valve (30). Upon expansion of the lugs (20) and pads (18) with assistance of the spring (17), the plunger meets the production tubing ID, while collapsing to pass through the safety valve (30) and not substantially affecting performance of the plunger.

As shown in FIG. 2F, the pads (18) and lugs (20) are interlocking such that movement of the lugs causes movement of the pads. In one embodiment, the lugs (20) are equipped with small pins (24) which travel within radial grooves in the pad mandrel to guide radial travel of the lug and prevent binding of the lug on the pad mandrel.

The top sub of the plunger may incorporate a valve mechanism, such as that described in U.S. patent application Ser. No. 12/027,062 filed Feb. 6, 2008, the contents of which are incorporated herein by reference, where permitted. Therefore, when the plunger is falling through the wellbore, the top valve of the plunger remains open, allowing any gas or liquid in the wellbore to pass through the plunger. The valve closes when the plunger lands on the bottom hole assembly, and production pressure causes the plunger to rise up, pushing production fluids above it to the surface. The spring (17) pressure pushes the lugs and pads radially outwards, to contact the production tubing surface.

In one embodiment, the plunger body comprises chamfered edges (22) at both top and bottom ends, which facilitates entry and passage of the plunger through the safety valve. The pads themselves are chamfered or rounded such that when the plunger enters the reduced diameter bore of the safety valve, the pads are physically retracted into the plunger, as is shown in FIGS. 2B and 2C. Upon exiting the safety valve, the spring (17) assists in expanding the pads once again, as is shown in FIGS. 2D and 2E. The plunger preferably has enough mass to allow easy passage downwards through the safety valve.

In this system and method, efficient use of the existing completion/tubing strings in a given wellbore may be made. The plunger (14) cycles through the tubing retrieved sub- surface safety valve (30). In this case, the safety valve (30) valve remains intact and operational whilst the fluid is produced to surface cycling the plunger (14) through the safety valve.

Due to the low reservoir pressures in a typical plunger lift application, small accumulations of fluid in the tubing string can dramatically decrease the inflow from the reservoir. This can be visualized by the discrepancy between the sale pressure and the casing pressure in FIG. 4. As shown with the production trend in FIG. 5, with the plunger in place and cycles controlled, the sales and casing pressure trendlines are tracking very closely indicating the flowing differential due to the accumulated liquid head in the tubing has been eliminated. This type of information and degree of control over the plunger lift system can be achieved by tracking and recording the plunger arrivals and catching and holding the plunger at surface using a programmed sequence in the wellhead controller. The present invention permits such programmed sequences with an active safety valve in place.

As will be apparent to those skilled in the art, various modifications, adaptations and variations of the foregoing specific disclosure can be made without departing from the scope of the invention claimed herein. 

1. A plunger lift production system comprising: (a) a production string comprising a bottom hole assembly, a production tubing having an inside diameter and including a safety valve having an inside diameter; (b) a plunger comprising a plunger body having an outside diameter which closely matches the inside diameter of the safety valve, and an expandable lug and pad assembly, which when expanded closely matches the inside diameter of the production tubing, and when not expanded closely matches the inside diameter of the safety valve.
 2. A method of producing fluids from a subterranean wellbore having a production string comprising a bottom hole assembly, a production tubing having an inside diameter and including a safety valve having an inside diameter, the method comprising the step of cycling a plunger within the production string through the safety valve, wherein the plunger comprises a plunger body having an outside diameter which closely matches the inside diameter of the safety valve, and an expandable lug and pad assembly, which when expanded closely matches the inside diameter of the production tubing, and when not expanded closely matches the inside diameter of the safety valve. 